Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Estimated total build cost
$120,000
Shell/structure cost
$66,000
Interior finish cost
$54,000
Psst — share this and help Rex grow
One click, a permanent link with your numbers baked in.
How to use this
- 1Enter adu area (sq ft).
- 2Enter price per sq ft ($).
- 3Enter shell (structure) share of cost (%).
- 4Read your estimated total build cost on the right — it updates as you type.
- 5Hit Share to keep the scenario or send it to someone.
About this calculator
A 400 sq ft ADU (accessory dwelling unit) is a common size for a backyard studio or in-law suite. Construction costs vary widely by region and finish level, so this calculator lets you set your own price per square foot and returns a total estimated build cost along with a breakdown between shell and finish work using an editable split.
Worked example
Using the values the calculator loads with:
Inputs
- ADU area: 400 sq ft
- Price per sq ft: 300 $
- Shell (structure) share of cost: 55 %
Results
- Estimated total build cost: $120,000
- Shell/structure cost: $66,000
- Interior finish cost: $54,000
What each field means
Inputs
- ADU area (sq ft)
- The adu area used in the calculation, measured in sq ft. Starts at 400 sq ft so you have a working example on load.
- Price per sq ft ($)
- The price per sq ft used in the calculation, measured in $. Starts at 300 $ so you have a working example on load.
- Shell (structure) share of cost (%)
- The shell (structure) share of cost used in the calculation, measured in %. Starts at 55 % so you have a working example on load. Accepted range: 20–80 %.
Results
- Estimated total build cost
- Returned as a money amount in US dollars and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Shell/structure cost
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Interior finish cost
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How much does a 400 sq ft ADU cost to build?
Costs vary heavily by region and finish quality, commonly ranging from $150 to $500+ per square foot — adjust the price field to a local contractor quote for accuracy.
What's included in 'shell' cost?
Shell typically covers foundation, framing, roofing, exterior siding, windows, and rough plumbing/electrical, while finish covers drywall, flooring, cabinets, fixtures, and paint.
Does this include permits and site work?
No, permits, utility hookups, and site prep can add significant cost on top of this estimate and vary greatly by jurisdiction.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). 400 Sq Ft ADU Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/400-sq-ft-adu-calculator
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/400-sq-ft-adu-calculator" target="_blank" rel="noopener">400 Sq Ft ADU Calculator — RevenueLab</a> (2026).</p>
Source: [400 Sq Ft ADU Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/400-sq-ft-adu-calculator) (2026).